Astronomy free-response question: Kepler's Third Law

Astronomy 10L Lab 12, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Students were asked the following open-ended question in an online pre-lab assignment, in order to prepare for a lab on determining the mass of Jupiter from the orbital periods and semi-major axes of its Galilean satellites:

[0.2 points.] In your own words, describe Kepler's third law. (Graded for completion.)

Students were graded for any non-blank response that demonstrates a serious (or not-so serious) attempt at completing the assignment.

Student responses
Sections 4161, 4162, 4163, 6544

Most responses were correct or nearly correct:

"The squares of the orbital periods of planets are directly proportional to the cubes of the semi-major axis of the orbits."
--K. D.

"The farther a planet is away from the sun, the longer time it will take for that planet to orbit the sun once."
--C. R.

"Which one was that... is that the one where all planets... I give up. The first one is about ellipses,... it has something to do with how long planets take to orbit the sun, I think."
--A. R. (1)

"the close ones go faster just like michelle kwan."
--M. M.

A number of responses discussed Kepler's first or second laws, Newton's laws of motion, or even laws of cooling:

"Equal distances are covered in equal times."
--A. B.

"It represents the motion of planets around the sun."
--E. D.

"An object in motion will remain in motion."
--J. A.

"Area to volume ratio! yea."
--S. M.

Some responses went in apparently just for kicks (but still get full "completion" credit):

"its a law that kelper made."
--A. R. (2)

"My words dont do this law Justice."
--R. G.

"Drawin' a blank."
--B. M.

"Keplers third law is never talk about keplers first two laws."
--D. F.